In recent years, in various technological fields, the global environment problem including global warming has become a matter of great concern. Under the situation, in a power-generating plant, it has become urgent to reduce the total emissions of carbon dioxide gas, and a newly built plant has been strongly required to have a facility capable of generating power with high efficiency. For example, in a thermal power generation boiler, a technique for increasing the temperature and pressure of steam to generate power with high efficiency has been adopted as an effective measure. The increase in temperature and pressure of steam leads to a rise in the tube wall temperature of superheater tube and reheater tube of boiler, so that the steel tube used for the boiler is required to have not only high-temperature strength but also resistance to high-temperature oxidation caused by steam. As a method for preventing steam oxidation of steel tube, various proposals have been made so far as described below.
(A) Technique of Performing Solution Heat Treatment after Formation of Worked Layer
Patent Document 1 discloses an invention relating to a method for producing a surface fine-grained stainless steel tube, in which method, after an austenitic stainless steel tube has been subjected to solution treatment, cold working such as shot blasting, grinding, and polishing is performed on the tube surface, and then predetermined re-solution treatment is performed. Patent Document 2 discloses an invention relating to a method for thermo-mechanical treatment, in which an austenitic stainless steel tube is subjected to cold working at a working ratio of 20% or higher, and then is subjected to solid solution heat treatment at a temperature rise rate of 2.9° C./s or lower.
Patent Document 3 discloses an invention in which a fine-grained layer having a thickness of 30 μm or larger is formed on the inner surface of an austenitic iron alloy tube by using grains finer than the grain size number of No. 7, and thereafter cold working at a working ratio of 20% or higher is performed and recrystallization treatment is performed. Patent Document 4 discloses an invention in which cold working is performed so that the hardness at a position 20 μm deep from the inner surface of an austenitic stainless steel tube is Hv320 or higher, and solution treatment is performed.
(B) Technique of Increasing Contents of C and N to Form Fine-Grained Micro-Structure Even after Solution Heat Treatment
Patent Document 5 discloses an invention relating to an austenitic stainless steel tube which has a fine-grained micro-structure having the grain size number of No. 7 or higher on the inner surface side of steel tube and whose fine-grained layer portion has a (C+N) content of 0.15% or more.
(C) Technique of Peening to Form Cold-Worked Layer
Patent Document 6 discloses an invention in which, after an austenitic stainless steel tube has been subjected to final heat treatment, peening is performed by blowing particles onto the inner surface of the steel tube. Patent Document 7 discloses an invention in which an austenitic stainless steel tube is subjected to peening treatment under predetermined conditions to form a worked layer having a thickness of 10 μm or larger. Patent Document 8 discloses an invention in which a tube body taken out of the existing boiler is subjected to chemical cleaning to descale the inner surface after heat treatment, and thereafter the inner surface of the tube body is subjected to shot blasting to form a cold-worked layer.
(D) Technique for Improving Adhesiveness of Scale
Patent Document 9 discloses an invention relating to a boiler steel tube excellent in steam oxidation resistance, which is produced by subjecting an austenitic stainless steel tube containing a rare earth metal to solution treatment, and has a peened layer formed by blowing particles onto the inner surface of the steel tube. Patent Document 10 discloses an invention relating to a steel tube which contains 9 to 28 mass % of Cr, and in which the maximum height of profile on the inner surface after cold working is 15 μm or larger, and further the difference in Vickers hardness between the inner surface layer and the wall thickness central portion of the tube is 100 or more.
(E) Technique for Subjecting Steel Tube to Cold Working at High Working Ratio
Patent Document 11 discloses an invention in which ultrasonic impact treatment is performed on the inner surface of a heat resistant ferritic steel tube or heat resistant austenitic steel tube containing 5 to 30 mass % of Cr. Patent Document 12 discloses an invention relating to an austenitic stainless steel tube which contains 16 to 20 wt % of Cr and whose inner surface is cold-worked, and more particularly relating to an austenitic stainless steel tube for boilers, in which the Cr concentration at a position near the inner surface of the steel tube is 14 wt % or higher, and the hardness at a position 100 μm deep from the inner surface of the steel tube is 1.5 times or more of the average hardness of base metal or Hv300 or higher. Patent Document 13 discloses an invention relating to a steel tube excellent in steam oxidation resistance, which has a worked layer having a high hardness of a steel tube containing 8 to 28 mass % of Cr.
(F) Technique for Improving Steam Oxidation Resistance of Heat Resistant Ferritic Steel
Patent Document 14 discloses an invention relating to a method for working a heat resistant ferritic steel, in which method a steel containing 9.5 to 15% of Cr is normalized and tempered to make the crystal grains and the micro-structure uniform, and thereafter a shot-blasted layer is formed by blowing particles onto the surface.